hose found in other parts of the world and of those confined to the
archipelago, are common to the several islands; and we may infer from
the present manner of distribution that they have spread from one island
to the others. But we often take, I think, an erroneous view of the
probability of closely allied species invading each other's territory,
when put into free intercommunication. Undoubtedly, if one species has
any advantage over another, it will in a very brief time wholly or in
part supplant it; but if both are equally well fitted for their own
places, both will probably hold their separate places for almost
any length of time. Being familiar with the fact that many species,
naturalised through man's agency, have spread with astonishing rapidity
over wide areas, we are apt to infer that most species would thus
spread; but we should remember that the species which become naturalised
in new countries are not generally closely allied to the aboriginal
inhabitants, but are very distinct forms, belonging in a large
proportion of cases, as shown by Alph. de Candolle, to distinct genera.
In the Galapagos Archipelago, many even of the birds, though so well
adapted for flying from island to island, differ on the different
islands; thus there are three closely allied species of mocking-thrush,
each confined to its own island. Now let us suppose the mocking-thrush
of Chatham Island to be blown to Charles Island, which has its own
mocking-thrush; why should it succeed in establishing itself there?
We may safely infer that Charles Island is well stocked with its own
species, for annually more eggs are laid and young birds hatched
than can possibly be reared; and we may infer that the mocking-thrush
peculiar to Charles Island is at least as well fitted for its home as is
the species peculiar to Chatham Island. Sir C. Lyell and Mr. Wollaston
have communicated to me a remarkable fact bearing on this subject;
namely, that Madeira and the adjoining islet of Porto Santo possess many
distinct but representative species of land-shells, some of which
live in crevices of stone; and although large quantities of stone are
annually transported from Porto Santo to Madeira, yet this latter island
has not become colonised by the Porto Santo species: nevertheless, both
islands have been colonised by some European land-shells, which no
doubt had some advantage over the indigenous species. From these
considerations I think we need not greatly ma